License for iOS will give you full access only to the iOS app. License purchased for macOS app activates full access to iOS and macOS together, but not vice versa.
If you made an in-app purchase that you downloaded from AppStore you can use this activation only on one iOS device simultaneously. And when will need to use the app on another device you can just press "Restore purchase" on the paywall screen and your license will immediately be activated on your other device. In that case, the previous license will be deactivated on the previous device.
Both of your devices should use the same Apple ID (iCloud account). Because any purchases in AppStore are attached to it.
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